Concentrations of d-lactate and its related metabolic intermediates in liver, blood, and muscle of diabetic and starved rats

This is a report investigating the methylglyoxal (MG) bypass in animals, by which d-lactate is produced from triosephosphate via MG. Rats were made diabetic using streptozotocin or starved for 72 h. d-Lactate and various metabolites related to it, such as l-lactate, pyruvate, methylglyoxal, glucose, and inorganic phosphate, were measured in the blood plasma, liver, and skeletal muscle of the rats. Diabetic and starved rats had significantly higher levels of d-lactate in plasma, liver, and skeletal muscle compared with the control group. In contrast, pyruvate levels in plasma, liver, and skeletal muscle was markedly lower than normal in diabetic and starved rats. l-Lactate level lowered markedly in plasma, liver, and skeletal muscle of starved rats and elevated in liver of diabetic rats. Differences between plasma l-lactate level for diabetes and control were not significant. MG level was significantly elevated in plasma and depressed in livers and muscles of starved rats as well as livers of diabetic rats. Hepatic glycerol content was markedly increased in those states. Enzyme activities related to d- and l-lactate, such as pyruvate kinase, phosphofructokinase, aldolase, and glyoxalase I, were measured in the livers of these rats. Pyruvate kinase activity decreased in these states, but other enzyme activities showed no significant changes. d-Lactate was much more excreted than l-lactate in the urine of diabetic and fasted rats compared with normal rats.